Project summary Dysfunction in generating and maintaining properly differentiated ciliated epithelia with motile cilia results in a number of human ciliopathies that can lead to hydrocephalus, congenital heart defects, respiratory infections, situs inversus and/or infertility. Whereas components involved in the assembly, structure and movement of motile of cilia have been identified, little is known about the transcriptional control of ciliogenesis. Here we seek to fill this gap by investigating the transcriptional regulation of the formation of a ciliated epithelium using the Kupffer's vesicle (KV) of the zebrafish embryo as a model system. This ciliated organ functions as the fish left-right organizer and develops, soon after gastrulation, from its precursors, the Dorsal Forerunner Cells (DFCs). We recently identified the transcription factors and cofactors known to mediate or to regulate the transcriptional outcome of the Hippo signaling pathway to be master regulators of DFCs differentiation. We discovered that they act upstream of major signaling pathways and key transcription factors known to control the formation of the KV and that they regulate, in the DFCs, the expression of epigenetic modifying enzymes. In particular, we found that Hippo transcription factors and cofactors control the activity of Nodal signaling during differentiation of the DFCs into KV, a previously unknown role for this pathway. We propose three aims to elucidate the mechanisms by which Hippo transcription factors and cofactors control the formation of the ciliated epithelium of the KV. First, we will characterize the transcriptional network that is directly and indirectly regulated by each of these factors. We will then perform functional analyzes on selected direct target genes to identify novel functions required for the formation of the ciliated epithelium of the KV. Second, we will explore the functional relationships between Hippo transcription factors and cofactors via rescue experiments and analysis of their physical interactions. We will challenge the current model of their interaction during normal development, previously established in a cancer cell culture system, and in which they are thought to act antagonistically. Third, we will investigate the mechanisms by which Hippo transcription cofactors control Nodal activity, for example by a transcriptional control of writers of DNA methylation marks. The completion of these studies will greatly advance our knowledge of the mechanisms by which a ciliated epithelium is programmed at the transcriptional level. In addition, by providing a new model of interaction between Hippo downstream mediators and regulators during normal development, and defining tissue-specific transcriptional control of epigenetics by Hippo transcription factors and cofactors, our results will greatly expand our knowledge of Hippo signaling pathway. Finally, insights gained about the generation and maintenance of a properly differentiated ciliated epithelium during embryonic development can be applied to the development of much-needed treatment for human ciliopathies.